Afmite, Al_3(OH)_4(H_2O)_3(PO_4)(PO_3OH)•H_2O, a new mineral from Fumade, Tarn, France: description and crystal structure

Abstract

The new mineral afmite, Al_3(OH)_4(H_2O)_3(PO_4)(PO_3OH)·H_2O, is triclinic with space group Pl and cell parameters a = 7.386(3), b = 7.716(3), c = 11.345(4) Å, a = 99.773(5), β = 91.141(6), y = 115.58(5)°, V = 571.6(3) Å^3 and Z = 2. It occurs, sometimes in association with matulaite and variscite, in fractures and solution cavities in shale/siltstone at Fumade, Tarn, France. The formation is probably largely the result of remobilisation and crystallisation during low-temperature hydrothermal activity and/or weathering and ground water activity. Afmite forms in cockscomb aggregates of diamond-shaped tablets on {001}, ubiquitously contact-twinned on {001} and also commonly twinned by rotation on [010] with {010} and {110} composition planes, forming star-like sixlings. The streak of the mineral is white, the luster is pearly, and the Mohs hardness is about 11/2. The mineral is flexible, but not elastic, has an irregular fracture and three cleavage directions: {001} perfect, {010} and {1Formula 0} good. The measured density is 2.39(3) g/cm^3 and the calculated density is 2.391 g/cm^3 based upon the empirical formula. Optical properties (white light): biaxial (+), α = 1.554(1), β = 1.558(1), y = 1.566(1), 2V_(meas). = 70(5)° and 2V_(calc) = 71°. Electron microprobe analyses provided Al_2O_3 40.20 and P_2O_5 38.84 wt% and CHN analyses provided H_2O 25.64 wt%, total 103.68 wt%. Normalized EMP analyses and water based on the structure yield Al_2O_3 36.41, P_2O_5 35.17 and H_2O 28.42, total 100.00 wt%. Infrared and Raman spectra were consistent with the PO_3OH, OH and H_2O as indicated by the crystal-structure determination. The strongest powder X-ray diffraction lines are [d_(obs)(Å),I_(obs),(hkl)]: 11.089,100,(001), 3.540,81,(013,112), 5.484,79,(002,101), 2.918,60(122), 3.089,33(113,201), 4.022,30,(102,112), 6.826,23,(010). The crystal structure, solved from twinned data, (R_1 = 10.4 % for 1262 F_o > 4σF reflections) consists of chains of AlO_6 octahedra parallel to [110] in which edge-sharing octahedral dimers share corners with individual octahedra. Both PO_4 and PO_3OH tetrahedra link the chains into sheets parallel to {001} and the PO_4 tetrahedra further serve to link two sheets together into a thick slab in which tetrahedral (T) and octahedral (O) layers alternate, forming a T-O-T-O-T sandwich. The linkage between these sandwiches in the c direction is via hydrogen bonding with isolated H_2O groups in the interlayer region. Afmite is closely related structurally to the turquoise-group minerals and specifically to planerite. The recently described mineral kobokoboite probably has a closely related sheet structure.

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